In the past, a filter of the kind that receives large signals comprises a first fixed filter 1, a second fixed filter 2 and a third fixed filter 3, each having a different operating frequency, connected in parallel between filter change-over switches 4 and 5, as shown in FIG. 18. A common terminal of the filter change-over switch 4 is connected to an input terminal 6, and a common terminal of the filter change-over switch 5 is connected to an output terminal 7, so that their change-over is executed through a change-over unit 9 by a signal from a change-over control terminal 8.
This structure of the prior art, however, has a drawback in that it may become large in size since it may require three fixed filters 1 through 3 as well as many circuitries such as two filter change-over switches 4 and 5, etc. On the other hand, a variable frequency filter consisting of a variable capacitance diode and an inductor may be used alternatively, but it occasionally produces disturbance waves as contrary to its object of being a suppressor of undesirable signals due to harmonics caused by distortion of the variable capacitance diode when a large signal is input.